Responses Of The East Asian Summer Monsoon To Aerosol Forcing In CMIP5 Models

Anthropogenic aerosol forcing has been considered to be an important factor driving the long-term variability of the East Asian summer monsoon?EASM?,but its underlying mechanism is still not fully understood.This study quantitatively distinguishes the fast and slow responses of the EASM to aerosol forcing using the outputs from 16 Coupled Model Intercomparison Project Phase 5?CMIP5?models.The mechanism of the EASM change due to aerosol forcing is evaluated from the perspective of upper-tropospheric temperature change.Then,the role of East Asian and non-East Asian?NEA?aerosol forcing in the fast response of upper tropospheric temperature over East Asia during summer and the EASM circulation to global aerosol forcing is examined using the global aerosol-climate online coupled model BCC?AGCM2.0?CUACE/Aero.The main conclusions are summarized as follows:?1?The CMIP5 multimodel ensemble results show that the global annual mean aerosol effective radiative forcing at the top of the atmosphere from 1850 to 2000 is-1.0 W m^-2.Changes in radiation fluxes caused by aerosols lead to the change of regional surface temperature,precipitation,and circulation over the East Asian monsoon region?EAMR?.The aerosol forcing results in a surface cooling of-0.7 K and an increase of 0.2 h Pa in surface pressure over the EAMR and the weakening of the EASM.Consequently,the precipitation rate over the EAMR is decreased,with the maximum reduction reaching 1.0 mm/day in the coastal areas of southern China.The aerosol–induced fast atmospheric response dominates the weakening of the EASM and the decreased precipitation over the eastern China.In the fast response,upper-tropospheric cooling in the mid-latitudes of East Asia during summer changes the circulation structure,thereby causing the weakening of the EASM.The slow ocean-mediated response to aerosol forcing partially offsets the weakening of the EASM over the eastern China in the fast response.The aerosol–induced changes of land–sea thermal contrasts further confirm that the decrease of upper-tropospheric land–sea thermal contrasts over East Asia in the fast response plays a key role in driving the weakening of the EASM.?2?The BCC?AGCM2.0?CUACE/Aero simulation experiments show that NEA aerosol forcing dominates the upper-tropospheric cooling in the mid-latitudes of East Asia during summer in the fast response to global aerosol forcing.The cooling is primarily due to the combined effects of strong eastward cold advection in the Northern Hemisphere mid-latitudes caused by increased aerosol loading in Europe and the resulting change in local meridional heat transport in East Asia.This is also the main reason for the decrease of upper-tropospheric land–sea thermal contrasts over East Asia in the fast response.The formation of the above-mentioned anomalous cooling center changes the zonal wind speed on both sides of it,and leads to the equatorward shift of the East Asian subtropical jet,thereby weakening the EASM.